In order to satisfy the stringent requirements that exist in many areas of application regarding the flame resistance of the materials used, plastics generally have to contain flame retardants. A large number of substances that are suitable as flame retardants are known and are also commercially available. Halogen compounds, phosphorus compounds, silicon compounds, zinc borates and metal hydroxides may be cited by way of example. By virtue of the often more advantageous secondary effects of fire in terms of smoke density, toxicity and corrosiveness and in particular for ecotoxicological reasons, the use of halogen-free flame retardant systems is preferred.
Flame-resistant polycarbonate compositions are known in principle and are used in a variety of areas of application, in particular in the electrical and electronics sector, in data technology, in construction and in the aircraft and railway industry. Phosphorus compounds, phosphorus-nitrogen compounds and silicon compounds are described in particular as suitable halogen-free flame retardants.
The use of phosphorus compounds, in particular of monomeric and oligomeric phosphoric acid esters as flame retardants in PC/ABS compositions is described for example in EP-A 0 345 522, EP-A 0 363 608 and EP-A 0 640 655. In addition to their suitability as a flame retardant additive, phosphoric acid esters display a plasticising action. The heat resistance of the compositions is therefore substantially reduced in some cases if they are rendered flame resistant.
The use of phosphorus-nitrogen compounds, such as phosphonate amines, phosphazenes and phosphoramidates as flame retardants in polycarbonate compositions is described for example in WO 01/18106, EP-A 1 116 772, EP-A 0 728 811, U.S. Pat. No. 6,414,060, U.S. Pat. No. 5,973,041 and WO 00/12612. In comparison to the aforementioned phosphoric acid esters these compounds are generally far less plasticising, but at the elevated temperatures that are typical in the compounding and processing of polycarbonate compositions they have a tendency towards thermal decomposition and/or degradation of the polycarbonate.
The use of silicon compounds, in particular of special silicones, as flame retardants in polycarbonate compositions is described for example in U.S. Pat. No. 6,001,921, WO 99/28387, WO 00/39217, WO 00/46299 and WO 00/64976. The silicones used may be incorporated into polycarbonate without any significant reduction in molecular weight, are thermally stable and have little or no plasticising action on the polycarbonate. The disadvantage of silicones is their comparatively low flame retardant efficiency, which limits their use to pure, i.e. non-impact-modified polycarbonate. The use of silicones alone to render blends such as PC/ABS blends flame retardant, at least by any economically viable means, is not possible.
Due to their lack of adequate efficiency, silicones are frequently used in combination with other flame retardants, such as the aforementioned phosphorus compounds for example. US 2002/0099160 A1 is cited here by way of example, wherein a combination of a special silicone and an oligomeric phosphoric acid ester is described as a flame retardant package for PC/ABS blends. The addition of silicone allows the amount of phosphate needed for a flame retardant effect to be reduced and hence its undesirable plasticising effect in some high-temperature applications to be limited, but not absolutely prevented.
JP-A 2001-247582 describes phosphorylated polyorganosiloxanes as low-plasticising flame retardants for polycarbonate and PC/ABS blends. Due to their poor compatibility with the polycarbonate and their in some cases high volatility at the conventional processing temperatures, the compounds used here as a flame retardant additive tend to bleed, which may lead to problems in injection molding in particular because of downtimes.
The object of the present invention was therefore to develop novel phosphorus-silicon compounds having high flame retardant efficiency, high thermal stability, low volatility and good compatibility with aromatic polycarbonates and to use them to provide flame-resistant polycarbonate compositions having improved processing characteristics.